The invention relates to a method and apparatus for producing sheet molding compound (SMC) parts, such as parts of automobiles or the like, by compression, and more particularly it relates to a method for producing SMC parts by compressing and coating in the mold to improve the productivity.
SMC sheets, which are composite material of reinforced thermosetting plastics, such as unsaturated polyester resins with fiber glass, have been recently used as parts of automobiles or the like instead of metal sheets. Heretofore, the SMC parts have been produced by conventional hydraulic press systems according to a typical operational stroke and time diagram as shown in FIG. 1. This process consists of charging a SMC sheet into a mold at A, closing the mold at B, gelling the SMC sheet, flowing and filling the SMC into the mold cavity at C, compressing and curing the SMC at D, opening the mold at E, and removing the formed SMC part at F. When the SMC parts are applied as parts of automobiles or the like, they serve to shorten the assembling process and lighten the weight, but they increase the cycle of time producing parts. Various research has been conducted to shorten the cycle time, but the special property of the SMC materials, such as curing, makes it difficult to shorten the cycle time to any great extent.
Furthermore, on most of the surfaces of the SMC parts produced by compressing there are formed small pinholes and sink marks or the like, and exterior parts, such as exterior body panels of automobiles, must be coated to improve their surface quality. When the SMC parts are coated during the forming process, their surface pit defects, such as pinholes and sink marks, can be removed and pit finding and pit fitting in the following process are eliminated to reduce the coating cost, but the cycle time is still further increased by coating, compressing and curing the coating parts.
The above cycle of the operational stroke and time diagram has been studied for the purpose of overcoming these shortcomings, and as a result, it has been found that the compressing and curing times make up more than one half of the cycle time, and the remainder of the time is consumed by the pressless steps of the operation, such as charging the SMC material, closing and opening the mold, and the like. Consequently, the resulting forming efficiency is not very high. In addition, during compressing and curing of the SMC sheets a predetermined large press force and corresponding power is required, whereas during the steps of the process other than the compression phase, the operation can be sufficiently performed by using a much smaller power requirement than that necessary during the compression phase. Furthermore, as shown in FIG. 1, in the former the operation can be performed within an extremely short stroke, but in the latter the operation requires a longer stroke. Thus, it is clearly understood that conventional SMC compression systems are uneconomically constructed and function at a low production rate.
As the SMC material is charged to a suitably heated mold, it is softened and fluidized, and upon heating becomes gelled. By applying a small amount of force, it can be easily made to flow and fill the mold cavity. As it is further heated, the material starts to cure and expand, and the compression power is gradually increased so as to suppress the expansion. As curing further proceeds, the material starts to contract, and a predetermined large force is applied and kept on the mold until the end of the curing. In this manner, the SMC parts are produced, but the long curing time requires the long cycle time as above mentioned. Furthermore, coating of the formed SMC part during the forming process further increases the cycle time.
Accordingly, if the compressing and curing operation and the other pressureless steps are separated, and the two phases of the operations are performed at the same time, the production rate can be doubled. However, as the SMC chemical reactions are continuously proceeding, the respective operations should be continuously performed, such as during the transferring of the mold, to form the SMC sheet into the desired shaped part.